1. Field of the Invention
This invention relates to loudspeaker systems.
2. Description of Related Art
GB-A-2 290 672 discloses a loudspeaker system comprising a bass unit, a mid-range unit, a treble unit, and a tweeter unit. Each of the units includes a respective loudspeaker drive unit. The mounting for the loudspeaker drive unit is such that there is substantially no rear reflecting surface behind the diaphragm of the loudspeaker drive unit. The pole piece of the respective magnet system of each loudspeaker drive unit is provided with an aperture through which, in use, sound from the rearward side of the diaphragm passes. Each of the loudspeaker drive units has a respective circular-section tube extending from the rear of the loudspeaker drive unit. Each tube contains sound-absorbent material such as glass fibre and tapers away from the associated loudspeaker drive unit.
The tapering tube is acoustically coupled to the rear of a loudspeaker drive unit to lead away and absorb sound waves produced at the rear of the loudspeaker drive unit. Such an arrangement is, however, satisfactory to the ear over only a certain bandwidth and that has the disadvantage that it is necessary to employ at least four such loudspeaker systems in combination, each to reproduce a respective part of the audio spectrum, if true high fidelity sound reproduction is to be obtained. Such combination systems are, of course, relatively expensive to manufacture.
EP-A-0 332 053 discloses an acoustic apparatus for improved bass sound reproduction which comprises a resonator, a vibrator, and a vibrator drive means. A Helmholtz resonator having an opening port and a neck serving as a resonance radiation unit is used as a resonator which is an acoustic radiation member. In the Helmholtz resonator, a resonance phenomenon of air is caused by a closed cavity (hollow drum) formed in a body portion and a short tube or duct constituted by the opening port and the neck.
FR-A-705 640 discloses a loudspeaker drive unit within a conical enclosure, which enclosure terminates in a very long tube rolled into a spiral. A series of holes are provided along the length of the tube and make connection to the external air. Sound waves in opposite phase are supposed to emerge from the holes and cancel each other out.
Objects and Summary
It is an object of the invention to overcome or mitigate the above-mentioned disadvantage of the prior art.
The present invention provides a loudspeaker system for reproducing signals between a first, lower frequency and a second, higher frequency of the audio spectrum, the system comprising:
a loudspeaker drive unit and
a tube acoustically coupled to the rear of the loudspeaker drive unit for leading away and absorbing sound waves produced at the rear of the loudspeaker drive unit, wherein:
the tube is acoustically coupled to the loudspeaker drive unit by means of a hollow resonant enclosure and the loudspeaker drive unit is mounted at an aperture in an external wall of the enclosure;
the tube communicates with the interior of the enclosure, extends outwardly from the enclosure, and is closed to the external surroundings along its length; and
there is a significant change in acoustic impedance where the tube communicates with the interior of the enclosure; characterized in that:
a crossover network is provided to define the first, lower and second, higher frequencies of sound reproduction of the system;
the fundamental resonant frequencies of the enclosure and the tube each lie between the first, lower and second, higher frequencies of sound reproduction of the system but the Helmholtz resonant frequency of the tube, as hereinbefore defined, lies below the first, lower frequency of sound reproduction of the system, and
means are provided to prevent the emergence of sound waves from the distal end of the tube.
The Helmholtz resonance frequency of the tube is here defined as the resonance frequency that occurs when the tube is open at both ends and the mass of air within it bounces on the stiffness of the air in the enclosure. The tube may, however, be closed at its distal end, in which case, the Helmholtz resonance frequency here defined can be determined by making the experiment of opening the closed end.
The invention is based on the realization that sound waves can bounce from side to the side in the tube of a loudspeaker system of the above-mentioned patent application so creating higher order resonances which can have an adverse effect on sound reproduction if they lie within the band that loudspeaker system is to reproduce.
When, in accordance with the present invention, the enclosure is provided and there is a significant change in acoustic impedance where the tube communicates with the enclosure, the enclosure acts, effectively, as a xe2x80x9cshort circuitxe2x80x9d to sound at the frequencies of the higher order resonances of the tube so that those resonances are not excited to any significant extent. The effect of the tube is effectively to remove energy from the resonances of the enclosure. Thus, the enclosure and tube act, so to speak, for the mutual benefit of each other. It is therefore possible to design the loudspeaker system to work over a frequency band that includes the higher order resonances of the tube.
Because the loudspeaker system can then be designed to work over a wider bandwidth, it becomes possible to use fewer loudspeaker systems in combination to cover the whole of the audio spectrum, so achieving a substantial saving in cost.
It is particularly to be noted that unlike loudspeaker systems of the prior art in which the Helmholtz resonant frequency lies within the band of operation of the system and is used to modify, by resonance, the frequency response within that band, the present invention requires the Helmholtz resonant frequency to be excluded from the band of operation of the system and to lie below it. The aim in the invention is to prevent resonance within the band of operation rather than to employ resonance within the band for a particular effect as was done in the prior art.
As is well known, loudspeaker systems for high fidelity sound reproduction have units operating over particular regions of the audio spectrum, namely, sub-woofer units for the very low bass frequencies, woofer or bass units for bass frequencies, mid-range units for the middle part of the audio spectrum, and tweeter units for high frequencies. The present invention can be applied with particular benefit to a mid-range unit.
The said Helmholtz resonant frequency may be less than one half the first frequency or less than one quarter the first frequency. Thus, in the invention there is co-operation between the enclosure and tube to overcome the effects of unwanted resonances in the tube within the pass band of the loudspeaker system rather than use of a Helmholtz resonance to extend the bass range of a loudspeaker system as in a conventional enclosure with a Helmholtz resonator.
The fundamental resonance frequency of the tube may be more than twice the first frequency.
The enclosure may be generally parallelepipedal, for example, of square or rectangular section. In such an enclosure, the resonant frequencies are related to each other by sine and cosine functions.
Alternatively, the enclosure may be generally spherical. In such an enclosure, the resonant frequencies are related to each other by a Bessel function.
The tube may taper from the point of communication with the interior of the enclosure and may taper away linearly or exponentially. For example, the tube may taper away exponentially with an exponential taper rate in the range xe2x88x928 to xe2x88x9214.
The distal end of the tube may be open and rely on attenuation within the tube to prevent the emergence of sound waves but preferably the distal end of the tube is closed to make certain that sound waves cannot emerge from the distal end of the tube.
Preferably, sound absorbent material is provided in the interior of the enclosure. By that means the resonance of the enclosure can be damped in order to optimize its effectiveness.
Preferably, sound absorbent material is provided in the interior of the tube. By that means, reliance does not have to be placed exclusively on the attenuating effect of a taper. When the distal end of the tube is closed, sound waves reflected from the closed end meet the sound absorbent material for a second time after reflection.
The tube is particularly effective when it has a diameter approximating to that of the loudspeaker drive unit and is of length at least equal to the diameter of the loudspeaker drive unit. In the above mentioned earlier application, it was preferred that the tube be at least six times the diameter of the loudspeaker drive unit and the fact that such a long tube is no longer preferable itself indicates the remarkable benefit obtainable by the use of the present invention.
The internal volume of the enclosure may be less than 25 times the internal volume of the tube, preferably less than 10 times, yet more preferably less than 5 times, or it may be less than 3 times the internal volume of the tube. The internal volume of the enclosure is preferably in the range 3 to 10, more preferably, 4 to 8 times the internal volume of the tube.
Preferably, the tube extends rearwardly substantially along the axis of the loudspeaker drive unit. Such an arrangement is particularly satisfactory acoustically, mechanically and aesthetically.
Preferably, the loudspeaker drive unit is located on a central axis of the enclosure. Again, such an arrangement is particularly satisfactory acoustically, mechanically and aesthetically.
Preferably, the enclosure has an internal volume in the range 6 to 10 liters. Such a volume works well for a mid-range loudspeaker system.
Preferably, the tube has a length between 24 and 36 centimeters. Such a tube length works well for a mid-range loudspeaker system.
The invention also provides a multi-way loudspeaker system comprising a plurality of systems according to the invention in combination, each system being arranged to reproduce a respective part of the audio spectrum.